Exhaust gas recirculation (EGR) has been used to lower emissions of internal combustion engines. Introducing EGR to engine cylinders can reduce engine pumping losses as well as formation of NOx. In WO2008/127755, a system is described having an EGR passage located downstream of a compressor on the intake side of an engine and upstream of a turbine on the exhaust side of the engine. The reference also describes a catalyst and particulate filter disposed in the exhaust system at a location downstream of the turbine. In one configuration, the reference describes flowing gases from a location downstream of the compressor at the intake side of the engine, to a location in the exhaust system upstream of the catalyst and particulate filter. The method described in the reference may be useful for diesel applications; however, introducing intake manifold gases to an exhaust system may have undesirable results for engines that operate with gasoline. Specifically, introducing oxygen upstream of a three-way catalyst for gasoline applications can increase tailpipe NOx emissions since three-way catalysts convert HC, CO, and NOx more efficiently around stoichiometric conditions.
Recently, direct injection gasoline engines have been shown to improve engine performance and to reduce transient air-fuel disturbances that may be caused by fuel adhering to the intake manifold and ports of an engine. However, at higher engine speeds and higher engine loads, particulates may form in engine exhaust. Under some conditions, formation of the particulates may be related to the short amount of time between when fuel is injected to the cylinder and when combustion is initiated by a spark plug. Specifically, there may be only a short opportunity for the injected fuel to completely vaporize and form a homogeneous mixture before combustion is initiated. If a homogeneous air-fuel mixture is not formed in the cylinder before combustion is initiated, pockets of stratification may form, and soot may be produced by combusting rich areas within the cylinder air-fuel mixture. Particulate filters have been proposed as one way to reduce emissions of soot.
The inventors herein have developed a method for regenerating a particulate filter without having to disturb the operation of a three-way catalyst. In particular the inventors have developed a method for regenerating a particulate filter, comprising: operating a direct injection gasoline engine having intake and exhaust systems; at least one cylinder of said direct injection gasoline engine combusting a substantially stoichiometric air-fuel mixture; and flowing gases from said intake system to said exhaust system at a location upstream of a particulate filter and downstream of a three-way catalyst.
By flowing gases from the intake system to a location in an exhaust system downstream of a catalyst and upstream of a particulate filter; soot held in the particulate filter may be oxidized without disturbing the processing of engine exhaust gases in an upstream catalyst. For example, during part-throttle conditions, engine feed gas emission consisting of HC, CO, and NOX can be converted to N2, CO2, and H2O by way of a three-way catalyst. However, these catalytic induced reactions are more efficient when engine feed gases are near stoichiometric conditions. In addition, soot accumulated in a particulate filter disposed in an exhaust system may be converted to CO and CO2 when sufficient oxygen and temperature are present at the particulate filter. Introducing oxygen from the intake system to a location in the exhaust system downstream of a catalyst and upstream of a particulate filter allows gases entering the catalyst to remain near stoichiometry, while also allowing oxygen to be present at the particulate filter.
The present description may provide several advantages. Specifically, the approach may improve engine emissions by allowing a catalyst to operate in an efficient operating window while at the same time regenerating a particulate filter. Further, the present method allows EGR to flow to the engine while a particulate filter is being regenerated. Further still, the rate of particulate matter oxidation can be regulated by controlling the flow between the intake system and the exhaust system from feedback output from an oxygen sensor located downstream of the particulate filter.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.